The world production of carbon fiber in 2010 was 40 kilo metric tons (KMT) and is expected to grow to 150 KMT in 2020. Industrial-grade carbon fiber is forecasted to contribute greatly to this growth, wherein low cost is critical to applications. The traditional method for producing carbon fibers relies on polyacrylonitrile (PAN), which is solution-spun into fiber form, oxidized and carbonized. Approximately 50% of the cost is associated with the cost of the polymer itself and solution-spinning.
In an effort to produce low cost industrial grade carbon fibers, various groups studied alternative precursor polymers and methods of making the carbon fibers. Many of these efforts were directed towards the sulfonation of polyethylene and the conversion of the sulfonated polyethylene to carbon fiber. But the methods and resulting carbon fibers are inadequate for at least two reasons. First, the resulting carbon fibers suffer from inter-fiber bonding. Second, the resulting carbon fibers have physical properties that are inadequate.
For example, U.S. Pat. No. 4,070,446 described a process of sulfonating high density polyethylene using chlorosulfonic acid (Examples 1 and 2), sulfuric acid (Examples 3 and 4), or fuming sulfuric acid (Example 5). Example 5 in this patent used 25% fuming sulfuric acid at 60° C. for two hours to sulfonate high-density polyethylene (HDPE), which was then carbonized. When the inventors used this method to sulfonate linear low density polyethylene (LLDPE), the resulting fibers suffered from inter-fiber bonding, and poor physical properties. Consequently, this method was judged inadequate.
In Materials and Manufacturing Processes Vol. 9, No. 2, 221-235, 1994, and in Processing and Fabrication of Advanced Materials for High Temperature Applications-II; Proceedings of a Symposium, 475-485, 1993 Zhang and Bhat reported a process for the sulfonation of ultra-high molecular weight (UHMW) polyethylene fibers using only sulfuric acid. Both papers report the same starting Spectra fibers and the same sulfonation process. The fibers were wrapped on a frame and immersed in 130-140° C. sulfuric acid and the temperature was slowly raised up to 200° C. Successful sulfonation times were between 1.5 and 2 hours. The fibers were removed at discrete intervals and washed with tap water, dried in an oven at 60° C. and carbonized in an inert atmosphere at 1150° C. Although good mechanical properties of the carbon fibers were obtained in this method, an expensive gel-spun polymer fiber was utilized and prolonged reaction times were used. As a result, we judge this method to be inadequate.
In Polymer Bulletin, 25, 405-412, 1991 and Journal of Materials Science, 25, 4216-4222, 1990 A. J. Pennings et al. converted a linear low-density polyethylene to carbon fiber by immersing fibers into room-temperature chlorosulfonic acid for 5-20 hours. This process would be prohibitively expensive from an industrial prospective due to the high cost of chlorosulfonic acid as well as the long reaction times.
In 2002, Leon y Leon (International SAMPE Technical Conference Series, 2002, Vol. 34, pages 506-519) described a process of sulfonating LLDPE fibers (d=0.94 g/mL) with warmed, concentrated H2SO4. A two-stage sulfonated system was also described, wherein “relative to the first stage, the second sulfonation stage involves: (a) longer residence time at a similar temperature (or a larger single-stage reactor at a single temperature); or (b) a slightly higher acid concentration at a higher temperature.” See page 514. Specific times and temperatures were not disclosed. In this reference tensile properties of the resulting carbon fibers were determined differently than is convention. Cross-sectional areas used for tensile testing were “calculated from density (by pycnometry) and weight-per-unit-length measurements” (pg 516, Table 3-pg 517). However, ASTM method D4018 describes that diameters should be measured directly by microscopy. After adjusting the reported tensile properties using the microscopy-measured diameters (Table 2, pg 517) new values were determined as follows:
ReportedReportedAdjustedAdjustedYoung'sTensileYoung'sTensileTrialEst.MeasuredModulusStrengthModulusStrengthStrain#diametersdiameters(GPa)(GPa)(GPa)(GPa)(%)229-1014.31050.903510.440.86269-1013.2n.d.1.54n.d.0.89NA279-1014.01341.34680.681.0
The methods disclosed in this reference produce carbon fibers having inadequate tensile strength and modulus.
In spite of these efforts, adequate methods of converting polyethylene based polymer fibers to carbonized polymers are still needed. Thus, disclosed herein are methods of making carbonized polymers (preferably carbon fibers) from a polymer, the methods comprising the sulfonation of the polymer to form a sulfonated polymer, subsequent hot solvent treatment of the sulfonated fibers, followed by carbonization of the polymer. These methods result in industrial grade carbonized polymers (preferably carbon fibers) having superior properties, when compared to those that were not treated with a hot solvent. These new methods work with all sulfonation methods.